1. Field of the Invention
The present invention relates to a coordinate position digitizing system and more particularly to such a system suitable for use in a diagram making system incorporating a tablet plate and a stylus, the position of the stylus on the tablet plate being detected to provide data for drawing lines and symbols.
2. Prior Art
In a diagram display system, a diagram or the like is drawn on a tablet plate with a stylus, to produce data for display on a display system. The input system comprises a tablet plate and a stylus, forming a coordinate position digitizing system, in which the position of the stylus, on the tablet plate, is detected as successive positions of the stylus.
Such a coordinate position digitizing system includes a plate or tablet, a stylus, apparatus for detecting the position of the stylus on the plate or tablet, and a drive circuit for supplying a voltage or current required for the tablet or plate, as well as a signal processing circuit for processing a signal obtained from the input system.
It is proposed to employ electrostatic means for detecting the position of the stylus on the tablet. Such an arrangment is illustrated in FIGS. 1 and 2 of the drawings, where a position determining plate 10 (forming a tablet plate) comprises a plurality of strip conductors Y1, Y2 . . . Ym, arranged on a first insulating layer 11, and extending parallel to each other at regularly spaced intervals. A plurality of strip conductors X1, X2 . . . Xn are arranged on a second insulating layer 12 covering the surface of the first insulating layer 11, extending parallel to one another at regularly spaced intervals, such intervals being the same as the interval for the conductors Y1, Y2 . . . Ym in the direction perpendicular to the conductors Y1, Y2 . . . Ym. A third insulating layer 13 covers the surface of the second insulating layer 12. On this layer 13, the position determining plate 10 is provided, and a stylus including the detecting electrode 31 is placed on this layer 13. The strip conductors X1, X2 . . . Xn of the position determining plate 10 have switches 22.sub.1, 22.sub.2 . . . 22.sub.n provided at the ends thereof, and which switches constitute a switch circuit 22 which forms a driving circuit section for cooperation with a shift register 24. The strip conductors Y1, Y2 . . . Ym have switches 23.sub.1, 23.sub.2 . . . 23m at one end thereof which constitute a switch circuit 23 forming a driving circuit section for a shift register 25. The switches of the switch circuits 22 and 23 are grounded at terminals A, and are connected to a common voltage supply terminal 21 via terminals B.
During a unit detecting period, the switches 22.sub.1, 22.sub.2 . . . 22n of the switch circuit 22 are supplied successively with equal length pulses, from the output terminals N1, N2 . . . Nn, in order to toggle the switches for connection of the strip conductors to the voltage terminal B instead of the grounded terminal A. Thus the strip conductors X1, X2 . . . Xn are supplied successively with the voltage Vcc present at the voltage supply terminal 21. During another unit detecting period, the switches 23.sub.1, 23.sub.2 . . . 23m of the switch circuit 23 are supplied successively with pulse signals of equal width from the output terminals M1, M2 . . . Mm, to toggle the switches for connection to the terminal B instead of the terminal A. Thus, the strip conductors Y1, Y2 . . . Ym are supplied successively with the voltage Vcc present at the voltage supply terminal 21.
If the spacing between center lines of adjacent strip conductors X1, X2 . . . Xn and the strip conductors Y1, Y2 . . . Ym is represented by Lp, then the strips of the plate are scanned at a scanning speed Ss=Lp/Ts in the X direction, and in the Y direction.
When the detecting electrode 31 of the stylus (FIG. 1) is placed on the position determining plate 10, it generates an output voltage Vo, which varies for successive short periods Ts as shown in FIG. 8A, as a result of the electrostatic capacity formed between the strip conductors supplied with the voltage Vcc on the detection electrode 31. The output voltage Vo takes a maximum value Vm when the voltage Vcc is applied to the strip conductor of the X or Y group which is closest to the position of the detecting electrode 31. In practice, the voltage Vo takes stepwise varying values as illustrated in FIG. 8, only when the voltage Vcc is applied to a strip conductor near to the location of the detecting electrode 31, and the output voltage is substantially zero when the voltage Vcc is applied to a strip conductor positioned far from the location of the detecting electrode 31.
The X and Y positions of the detecting electrode 31 on the position determining plate 10 may be derived from the output voltage Vo. Specifically, the output voltage Vo is supplied to a tuning circuit amplifier which provides an output signal Sb of a predetermined frequency with a maximum amplitude at a time delayed by an instant after the time in which the voltage Vo takes its maximum level, as illustrated in FIG. 8B. The signal Sb is supplied to a Schmitt trigger circuit, and a signal Sz is derived therefrom, which signal falls from a high to a low level at the instant at which the signal Sb crosses a predetermined level Vs, corresponding to the threshold level of the Schmitt trigger. The output Sz rises from a low to a high level at the instant at which the signal Sb crosses ground potential, corresponding to the negative-going Schmitt threshold level, as illustrated in FIG. 8C.
The time period Td (FIG. 8C) extends from the instant t1 in which the voltage Vo is first supplied to one of the strip conductors X1, X2 . . . Xn or Y1, Y2 . . . Ym, until the instant t2 in which the signal Sz falls below ground level after having crossed above level Vs, corresponding to the time required for scanning from one end of the plate 10 to the position of the detecting electrode 31 at the scanning speed referred to above, in the X or Y direction, respectively. Thus the time interval Td corresponds to the distance from one end of the position determining plate 10 to the position of the detecting electrode 31. The time interval Td is detected by counting clock pulses having a pulse repetition rate substantially shorter than the short period Ts, using a counter, beginning with the instant t1 up until the instant t2. The content of the counter at the instant t2 is provided as an output corresponding to the position of the detecting electrode 31, on the position determining plate 10, in the X or Y direction, respectively, thus producing coordinate data for the detecting electrode 31 on the position determining plate 10.
In the arrangement described above, when the detecting electrode 31 of the stylus is moved on the position determining plate 10, in drawing a diagram or the like, a static electric charge may be generated from the sliding of the detecting electrode 31 on the insulating layer 13 which forms the upper surface of the position determining plate 10. This static charge may be loaded on the insulating layer 13, or, a static charge may be generated from the sliding of the clothing or the like of the operator moving the stylus on the insulating layer 13 so as to be loaded on the insulating layer 13. Such a static charge may be discharged through the detecting electrode 31. This causes a noise signal to be induced at the output of the detecting electrode 31, which may result in erroneous operation. Noise can also be produced in the output circuit, through induction, as a result of nearby electrical apparatus, or from the proximity of a human body to the detecting electrode 31.
When noise is present in the output of the detecting electrode 31, the normal voltage Vo as described above cannot be obtained and the true voltage level may be obscured by noise. For this reason, the time from the instant at which the voltage Vcc is first supplied to a strip conductor X1 or Y1, at one end of the position determining plate 10, to the time of rising or falling of the signal Sz, may not correspond to the distance from the end of a position determining plate to the position of the detecting electrode 31. In this event, the accurate detection of the position of the electrode 31 cannot be obtained. Thus erroneous coordinate data is produced for the position of the detecting electrode 31.
It is desirable to provide an apparatus and method which can generate digital data corresponding to coordinate position when a plurality of strip conductors are supplied successively with a given voltage, the voltage of each strip conductor being detected by detecting means which provides a detected output for corresponding coordinate data corresponding to the position of the voltage detecting means on the position determining plate, whereby a positional detection output is generated depending on such coordinate data, while eliminating the effect of erroneous coordinate data generated due to noise components, so that a correct position detection output is developed.